Vol. 54.] DIVISIONS or SO-CALLED ' JURASSIC ' TIME. 447 



essentially 'Jurassic' ammonite-fauna did not begin to hold dominant 

 sway until after the sedimentary change had been completed, and 

 that ammonites of a different stock lived while the change was in 

 progress (and in some districts after it was completed), occupying the 

 new areas which the changed conditions had opened up for them. 



What therefore it is justifiable to say is this — that during the 

 last few hemerse of the 'Triassic' Period there occurred certain 

 local physical changes, as a consequence of which the sea occupied 

 the areas of certain large inland lakes ; and that thus the ammonites 

 of the ' Triassic ' Period could extend themselves over a wider area. 

 So it is that the ammonite invasion makes a noticeable feature in 

 the fauna of certain districts, and, being connected with a lithic 

 change which has preceded it by a longer or shorter period in 

 different cases, to it has been ascribed an undue prominence. But 

 such a faunal feature, simply an accident of colonization owing to 

 the opening up of new territory, is not of universal chronological 

 importance, especially when the chronological data depend on the 

 biological features connected with the rise and fall of particular 

 ammouite-families. 



Such, then, are the considerations which have induced the proposi- 

 tion — that if the biological affinities of the ammonites be correctly 

 understood, the ' Jurassic ' Period should be regarded as com- 

 mencing at the beginning of the rotiformis hemera, and its first 

 subdivision should be the Asteroceratan Age (time of Sinemurian 

 Stage). 



II. The Ages oe the ' Jurassic ' Period. 



It is proposed to divide the ' Jurassic ' Period into two Epochs 

 — Arietidan and Stepheoceratidan — for reasons which will 

 be seen presently. The Arietidan Epoch is divisible on biological 

 grounds into four Ages, in the following manner : — 



(a) Eirstisthe Asteroceratan Age, comprising seven hem erae; 

 and the Arietidae, as now defined, excluding the Psiloceratidae and 

 Echioceras, dominate it entirely. They die away with some 

 suddenness during the hemera oxynoti; and very few species of 

 the family survive into the next Age. 



(b) The succeeding space of time is the Deroceratan Age, and 

 some strata called Lower Lias, besides all those called Middle Lias, 

 were deposited during its continuance. The Deroceratan Age com- 

 prises seven hemerse ; but there is no permanent predominance of 

 any particular family or genus during that time. 



In the first hemera the species of Echioceras are dominant. 

 Hitherto this genus has been regarded as one of the Arietidae ; but 

 it is very obvious from the simplicity of its suture-line, from the 

 uncarinate or merely subcarinate periphery of EcJiioceras raricos- 

 tatum, from the depressed and not compressed whorls of this 

 species, that a new departure has commenced, and that a new, less 

 developed stock has come into prominent existence The ancestors 

 of Echioceras must on these grounds be sought, not among the 

 carinate compressed-whorled Arietidae, but in a non-carinate, 



a.J.G. S, No. 215. 2 1 



